1. Field of the Invention
The present invention relates generally to the field of illumination. More particularly, the present invention relates to vapor-filled or gas-filled lamps for analytical instrumentation, such as interferometric endpoint detectors for use in semiconductor processing machinery.
2. Background Information
The manufacture of semiconductor devices uses many types of operations including etching, deposition, heating, ion implantation, and polishing. These operations are generally automated and are performed inside vacuum chambers under controlled conditions. Of course, sensors are necessary to carry out automated control of these operations.
Most modern etching equipment make some provisions for endpoint detection, i.e. detection of etch-through in a desired layer. One approach that is useful with transparent and semi-transparent layers (one example being silicon dioxide (SiO2)) is to use the principles of light interferometry. With light interferometry, a beam of light having some degree of coherence (laser or other light source that is at least short-term coherent) is directed at the layer being etched and a reflected portion of the beam is detected by an appropriate photodetector. Some of the incident beam will be reflected from the top surface of the layer and some of the beam will be reflected from the bottom surface of layer. These two reflections will either constructively or destructively interfere with each other, creating a characteristic etching curve (typically periodic) as the layer is etched away. When the etching curve changes or flattens out, the layer has been etched through and endpoint has been detected.
Rather than waiting for the curve to flatten out, a more sophisticated method (often used for etch of polysilicon) anticipates the endpoint by counting the number of oscillations of the sinusoidal etch curve (corresponding to the interference fringes) and stops the etching just short of the layer interface. This permits change over to use of a different mix of etch gasses to provide for a more discriminating etch at the end of the etch step.
A number of interferometric endpoint detection schemes have been devised, the above-described procedures being examples of many that are useful. A common feature of the interferometric techniques is the need for a coherent or partially coherent light source. In many instances, vapor lamp light sources are preferred over lasers. There are a number of reasons for this. Vapor lamp light sources are cheaper and more available than lasers and generally have a lower Mean Time Between Failure (MTBF). Additionally, a short-term coherent source is preferred over a long-term coherent source for some interferometric endpoint detection methods (such as those described above) for the reason that the less coherent source actually has the effect of isolating the parameters that are of interest and being relatively insensitive to noise parameters (e.g., movement of the wafer holder due to thermal effects).
Besides semiconductor process monitoring uses, mercury vapor lamps are also commonly used as stable light sources in the art of analytical instruments in general, particularly for providing light in the visible and ultraviolet ranges.
A stabilized vapor lamp light source is one that is engineered to provide light at a desired intensity and wavelength so that the intensity at the selected wavelength does not vary beyond a specified range of intensities. Stabilized light sources are useful in a wide range of analytical instrumentation applications. Analytical instruments that utilize a stabilized light source can optically detect anomalies in blood (such as AIDS), as well as concentrations of gaseous and metallic molecules and atoms (mercury, lead, arsenic, selenium, etc.), either as materials used in an industrial process or as environmental pollutants. These applications require that the lamps closely adhere to specified illumination intensity levels over time.
The presence of a metal in an environment is detected by observation of the amount of absorbence of spectral emissions from the same metal contained in the vapor lamp that occurs as the emissions pass through an optical cell containing atoms of this same metal. This arrangement has been used successfully for detecting and measuring very small quantities of mercury, zinc, and cadmium. The reason that sources incorporating these metals are utilized is that they operate within very broad thermal operating conditions.
Mercury vapor lamps (as well as those containing zinc or cadmium) are standard sources of ultra violet light and have evolved to the extent that they are reliable in maintaining the output level (i.e., the illumination intensity). This has been accomplished by a combination of improvements including mounting a lamp in an aluminum heat sink block so that the ambient conditions do not influence the temperature of the lamp itself, because temperature variations can otherwise influence the output level of the lamp. Another means for stabilizing the output has been to regulate the current through the lamp.
A stable light source, such as a mercury vapor lamp, has been positioned within a metal block, with a heater for the metal block, and a heater control for maintaining the temperature of the block constant. Light sources embodied according to this configuration are capable of maintaining a stable intensity at 254 nm of about xc2x10.25 percent. For additional information on light sources of this sort, refer to U.S. Pat. No. 3,457,454 to Boland. The Boland ""454 reference is incorporated herein by reference in its entirety for all purposes.
A stable light source, such as a mercury vapor lamp, has also been positioned within a metal block, with a heater for the metal block, and a control system that regulates not only heat but also current level supplied to the lamp. The control system includes an optical loop with a sensor to detect light output by the lamp and passing through a filter. Light sources embodied according to this configuration are capable of maintaining a stable intensity of better than 0.2 percent maximum peak-to-peak variation. For additional information on light sources of this sort, refer to U.S. Pat. No. 5,834,908 to Boland et al. The Boland ""908 reference is incorporated herein by reference in its entirety for all purposes.
In laboratory uses of instruments utilizing mercury vapor lamps (filled with Hg alone or Hg mixed with various other fill gasses) there is always a need by the end user for higher intensity outputs at specific wavelengths, and for close control of the relative intensity of the mercury and gas fill spectral line emissions.
Thus, what is needed is a light source having stability performance that exceeds that possible with known vapor lamp sources.
In particular, the vapor lamp stability controls according to the prior art are not suitable for providing precise control of output light intensity when the output light is coupled via a light guide such as an optical fiber bundle. That is because the optical sensor senses light according to a numerical aperture that is different from the numerical aperture at the end of the light guide where the output light is coupled into the light guide. This mismatch of numerical apertures between the light guide and the feedback sensor causes the feedback signal to be insensitive and/or inaccurate.
Thus, what is also needed is a vapor lamp light source that provides stable output intensity when the output is coupled via a light guide.
One feature of a light source according to the present invention is that its structure is very mechanically stable. Another feature is the use of accurate optical sampling to provide a sensitive feedback signal for the control system of the light source. Yet another feature is that the output light intensity of the light source is maximized.
It is an object of the present invention to provide a light source having stability performance that exceeds that possible with known vapor lamp sources.
It is another object of the present invention to provide a vapor lamp light source that produces high output from a compact package.
It is yet another object of the present invention to provide a vapor lamp light source that provides stable output intensity when the output is coupled via a light guide.
It is a further object of the present invention to provide a vapor lamp light source wherein a feedback light signal is sampled according to a numerical aperture that is the same as the numerical aperture of a light guide accepting the output of the light source.
It is a still further object of the present invention to provide a vapor lamp light source wherein a process controller is provided with a signal that indicates the status of the light source.
A further object of the present invention is to provide a way of controlling semiconductor fabrication processes to account for situations where a light source for a process monitoring instrument fails.
An additional object of the present invention is to provide a semiconductor processing system that incorporates an interferometric endpoint detection structure having an ultra-stable light source.
Some of the above objects are achieved by a light source having a substantially constant output level. The light source includes a unitary housing member having a light guide connection disposed at one end, and a vapor lamp positioned securely via the unitary housing member. A reflector is positioned securely via the unitary housing member to concentrate light output by the vapor lamp into the light guide connection. The light source also includes a reference detector positioned to receive a portion of the light output by the vapor lamp and generate a feedback signal, and a power supply connected to supply power to the vapor lamp in an amount controlled according to the feedback signal.
Others of the above objects are achieved by a light source having a substantially constant output level. The light source includes an optical fiber bundle having an end, a vapor lamp positioned near the end of the optical fiber bundle, and a reflector positioned to concentrate light output by the vapor lamp into the end of the optical fiber bundle. A beam splitter is positioned along a first optical path extending between the vapor lamp and the end of the optical fiber bundle. The light source also includes a reference detector to receive a portion of the light output by the vapor lamp and generate a feedback signal, the reference detector being positioned along a second optical path extending from the beam splitter angled away from the first optical path, wherein the second optical path is matched to the first optical path. A power supply connected to supply power to the vapor lamp in an amount controlled according to the feedback signal.
Some of the above objects are achieved by a monitoring method for use in a semiconductor fabrication process. The monitoring method includes a step of monitoring one or more parameters relevant to the fabrication process via a measurement instrument having a light source, as well as a step of monitoring a data output node of the light source for occurrence of an alert signal indicating the light source is in a failure mode. A signal perceivable by an operator is provided in the event that the alert signal occurs.